Something like "Thank you, thank you for releasing me from this human-body-shaped-prison, thanks to you I can finally reclaim my throne in hell and continue my Reign of Terror!"
Or maybe more like "Congratuations, you have freed the Demongod from his human-body-shaped prison and are at the base of the destruction of countless worlds, the genocide on and enslavement of even more people. Are you happy now? Cause I sure am!"